A UWB antenna apparatus is known in the conventional art as an antenna apparatus capable of using the frequency band characterized by an ultra wide-band. The UWB in the sense in which it is used here refers to the ultra wide-band in a wireless transmission system using a wide-band frequency width (several GHz through several tens of GHz) wherein a short pulse of 1 ns or less is employed. The UWB antenna apparatus is characterized by ultra low output transmission and very small interference with other wireless communications.
For example, the UWB antenna apparatus 50 is known in the conventional art, as shown in FIG. 11. This antenna apparatus is provided with a pair of radiating plates (conducting plates) 51, 51. The radiating plates 51 are semicircular in plan view, and are arranged at a space having a predetermined width while the arc vertexes of the radiating plates 51 are kept face to face with each other. The arc vertex of each of the radiating plates 51 is provided with a power supply section 54 that is connected with a coplanar strip line as a balanced line 53. When a predetermined current is fed from the balanced line 53 to the power supply section 54, the radiating plates 51 resonate in response to the current, with the result that radio wave is emitted from one or both ends thereof.
Still another antenna apparatus known in the conventional art is an antenna apparatus wherein the available bandwidth is increased by using the self-similar radiating plates shown in FIG. 12. The radiating plates of this antenna apparatus are self-similar and have an isosceles triangle in plan view, and hence this antenna apparatus is characterized by the principle of self-similarity which is independent of the frequency, in such a way that resonance is possible even in the high frequency area. In FIG. 12, the radiating plate of FIG. 12 (a) is greater in size than that of FIG. 12 (b). Their available bands are indicated by BWa and BWb, respectively.
One of the conventionally known indicators showing the characteristics of the antenna apparatus is the VSWR characteristic curve represented in a chart wherein the VSWR (Voltage Standing Wave Ratio) value is plotted on the vertical axis, and frequency is plotted on the horizontal axis. The VSWR characteristic curve is designed to assume the minimum value in the frequency wherein the radiating plates resonate. The frequency at which the radiating plates resonate is determined in proportion to the distanced from the power supply section to one end of the radiating plate. Accordingly, the VSWR characteristic curve differs according to the size of the radiating plate; thus, as the size of the radiating plate is greater, the minimum frequency for resonance is lower. Generally, the frequency band wherein VSWR value ≦2.0 corresponds to the band where the antenna apparatus can be employed. The self-similar antenna apparatus is designed in such a way that, as the minimum frequency is lower, the available band has a greater bandwidth.
Thus, to implement the antenna apparatus having a wide-band characteristic, it has been essential in the conventional art to increase the size of the radiating plate in order to reduce the minimum frequency. Further, the UWB antenna apparatus uses an increased band frequency width, and this makes it difficult to achieve resonance with a great number of frequency components contained therein. To put it another way, as the frequency of the radio wave to be sent and received is increased in bandwidth, the designing of the antenna apparatus becomes more difficult. This problem remains unsolved in the conventional art.
In an effort to solve this problem, an antenna apparatus has been developed and disclosed in the Patent Document 1. This is an antenna apparatus of an increased level of freedom having a wideband characteristic. In this antenna apparatus, a power supply section is arranged at a predetermined position of the space between radiating plates. The current fed from the power supply section is transmitted in such a direction that a self-similar shape can be easily created. Thus, a wideband characteristic is obtained.
Patent Document 1: Unexamined Japanese Patent Application Publication No. 2005-117363